All-optical signal processing and telecommunication devices are presently being developed to exploit the high speed, immunity to electromagnetic interference, very large bandwidth and capacity, and low loss, distortion and crosstalk characteristics of guided-wave lightwave technology, particularly in forms using optical fiber. "All optical" means the devices not only have optical inputs and outputs, but all intermediate signal processing is accomplished optically rather than electrically. All-optical devices are more desirable than opto-electrical devices which convert an optical signal into an electrical analog because a signal remaining in the single optical domain can make best use of the inherent advantages of lightwave technology. In addition, all-optical systems generally require less components and are less complex than their opto-electric counterparts performing similar functions.
One of the first areas to develop in all-optical signal processing was optical amplification with the advent of semiconductor optical amplifiers and rare-earth-doped amplifying optical fibers such as erbium-doped optical fibers. Rare-earth-doped amplifying optical fibers exhibit low noise, relatively large, polarization-independent bandwidth, reduced cross-talk, low insertion loss, and are relatively inexpensive to fabricate. Thus, all-optical amplifiers offer substantial performance advantages over competing opto-electrical amplifier technology.
Clearly, there is a need for more all-optical devices, beyond optical amplification, to further exploit the inherent advantages of lightwave technology in digital applications. For example, while electrical flip-flop devices have existed for many years, an all-optical flip-flop device has not yet been realized. An all-optical flip-flop would facilitate the implementation of all-optical sequential logic and bit-storage devices. Present optical bit-storage devices using electrical intermediate signal processing and fiber delay lines, while operating satisfactorily in certain applications, are not suitable for other applications due to the inherent limitations possessed by all opto-electrical devices.